The present invention relates to a developing method and an apparatus therefor of the type causing a developer carrier to carry and transport a one-component developer to a developing region where the developer carrier faces an image carrier so as to develop a latent image electrostatially formed on the image carrier. More particularly, the present invention relates to a developing method and an apparatus therefor which develops a latent image by use of a developer carrier capable of forming microfields thereon.
A developing device of the type using a powdery dry developer is extensively used with an electrophotographic copier, laser beam printer, facsimile transceiver or similar electrophotographic image forming equipment which electrostatically forms a latent image on an image carrier such as a photoconductive element and develops it by a developer. The powdery developer is available as a two-component developer which is the mixture of a toner and a carrier or a one-component developer which does not contain a carrier. Although a developing device using the two-component developer reproduces attractive images relatively stably, the carrier is apt to deteriorate and the mixture ratio of the toner and carrier is apt to change. This results in troublesome management of the apparatus and a bulky construction. For this reason, a developing device which uses the one-component developer free from the above problem is attracting much attention. The one-component developer is implemented with the toner only or with the toner and an auxiliary agent for controlling the polarity and amount of charge. The toner in turn is implemented as a magnetic toner containing magnetic power therein or a nonmagnetic toner which does not contain it. Since a magnetic body is usually opaque, color image, whether it be full-color or multicolor, developed by the magnetic toner does not appear sharp. Therefore, it is preferable to use the one-component developer constituted by the non-magnetic toner when it comes to color images.
In a developing device implemented with a one-component developer, a developing roller or similar developer carrier carries the developer thereon and transports it to a developing region where the developer carrier faces an image carrier. In this region, the developer develops a latent image electrostatically formed on the image carrier. A prerequisite with this type of developing device is that a great amount of sufficiently charged toner being fed to the developing region in order to insure high quality images having predetermined density. When the magnetic toner is used, a sufficient amount of one-component developer may be deposited on the surface of the developer carrier by magnets. However, the non-magnetic one-component developer is immune to magnetism, so that transporting a great amount of developer to the developing region is difficult.
Various implementations have been proposed in the past for eliminating the above problem. For example, a developing device disclosed in Japanese Patent Laid-Open Publication No. 43767/1986 has a developer carrier covered with an insulative dielectric layer, and a sponge roller or similar developer supply member held in pressing contact with the dielectric layer. The developer carrier and the sponge roller are charged to opposite polarities by friction. A non-magnetic one-component developer charged to the opposite polarity to the dielectric layer is electrostatically deposited on the dielectric layer and transported to a developing region. A drawback with this scheme is that the electric field developed in the vicinity of the surface of the dielectric layer is not intense enough to deposit a great amount of toner on the surface of the developer carrier and, therefore, the developer available in the developing region is short. In this condition, forming a developed image or toner image with high density is not easy. To eliminate this drawback, the developer carrier is moved at a speed twice or more higher than the moving speed of the image carrier. This, however, brings about another problem that the density of a solid image formed on the image carrier becomes unusually high in a trailing edge portion of the image with respect to the moving direction of the image carrier, resulting in poor image quality.
Another conventional developing device generates an electric field between the developer carrier and the image carrier in a direction for electrostatically transferring the non-magnetic one-component developer toward the developer carrier. Such an approach, however, also fails to deposit a sufficient amount of developer on the developer carrier.
Japanese Patent Laid-Open Publication No. 51841/1979 teaches another approach which uses a developer supply member for positively causing the non-magnetic developer to electrostatically deposit on the developer carrier. Specifically, after the developer carrier has moved away from the developing region, the non-magnetic one-component developer remaining thereon is scraped off. Then, the surface layer of the developer carrier is applied with a charge by corona discharge. The developer supply member positively and electrostatically deposits the non-magnetic developer on the charged surface of the developer carrier. With this approach, it is impossible to increase the amount of developer carried on the developer carrier and, therefore, to feed a great amount of toner to the developing region.
The developer carrier may be provided with undulations on the surface thereof so as to fill them with the non-magnetic one-component developer, as disclosed in Japanese Patent Laid-Open Publication No. 53996/1985. While such a configuration may be successful in increasing the amount of developer to reach the developing region, such a developer contains a substantial amount of toner whose charge is short and, therefore, cannot produce high quality images.
Further, Japanese Patent Publication No. 9711/1980 proposes a developing device having a developer carrier made up of a conductive support member, an insulating layer provided on the support member, and a conductive lattice member provided on the insulating member. The insulating layer is exposed to the outside through numerous openings formed through the lattice member. A voltage opposite in polarity to a developer is applied between the lattice member and the support member to generate microfields, so that a great amount of developer may be deposited on the surface of the developer carrier by the microfields. However, such microfields are not attainable without resorting at least an exclusive external power source, resulting in a complicated construction. Other approaches for generating microfields are taught in U.S. Pat. Nos. 3,739,748 (Rittler et al), 3,645,618 (Lancia et al), 3,759,222 (Maksymiak et al), and "Microfield Donors for Touchdown Development" by P. G. Andrus et al, SPSE 2nd International Conference on Electrophotography, October 1973.